The structure of the topside ionosphere deduced from Alouette data

Alouette II topside sounding data have been recorded at the Kashima station of the Radio Research Laboratories, Japan, and some preliminary results obtained from analysis of the topside sounding data during the one year period from October, 1966, to September, 1967, are reported. The plasma frequency at the satellite has been determined from several characteristic frequencies of each topside ionogram obtained from Alouette II data. The average nighttime structure of the ionosphere has been derived from plasma frequencies measured at various satellite positions over the data acquisition area of the Kashima station. The results are presented in terms of the electron-density distributions versus altitude and geomagnetic latitude. Some ionospheric parameters have been determined by fitting theoretical electron-density profiles to the observations. Theoretical electron-density profiles obtained on the assumption of constant plasma temperature (average of the electron and the ion temperatures) have been compared with those obtained for the case when the height gradient of the plasma temperature is put equal to 0.5 °K km. The comparison shows that the difference in the theoretical electron-density profiles between the two cases is not too serious, and hence, the theoretical electron-density profiles for the constant plasma temperature model were used to determine the ionospheric parameters. The results show that the probable thermopause temperature (i.e., neutral particle temperature at the base of the exosphere which is assumed to lie at the 600-km level) varies from about 1000 to 1200°K in the lower latitude region, to about 1500°K in the higher latitude region, and further that the plasma temperature (assumed constant with altitude) varies from about 1000°K in the lower latitude region to about 2500°K in the higher latitude region.